Hi I'm looking for a specific manga about a male protagonist who is taken in by a demonic cult and trained as an assassin. The following are spoilers.

He is part of a group of children trained in some kind of underground cave. Unbeknownst to them is that they are meant to be a sacrificial squad. They are to succeed in their mission and then be killed by their organization. The organization was employed by the Enmei sect? I think. It's largely a group of women. On the surface the Enmei sect and another sword sect are rivals who are forming a political alliance through the marriage of their disciples, but in the shadows the Enmei sect leader wants to kill the promising disciple of their rival and the marriage is a pretense.

Unfortunately for all the parties involved, the protagonist against all odds manages to kill his target, even though most of his fellow trainees die. The two sects also destroy the organization that trains the protagonist. Eventually his actions lead to the two sects suspecting each other and causing a war between them.

The protagonist is caught by an elder of the Sword Sect and thrown into a pit of poisonous snakes to die. Somehow the protagonist survives and is changed by the experience. His hair has turned white and his eyes become red with slits. His theme becomes a white snake. After he emerges from the snake cave, he decides to take revenge on the two sects.

As an assassin he slowly decreases the two sects numbers and plants suspicion between the sects. Eventually the sects do find out about him and he leads them into a trap. He chooses a location with hundreds of traps that severely diminishes his enemies' numbers. Despite the two sects having numerous people, he manages to instill fear into both sects after he kills the Enmei sect leader and sword sect elder. It gets to the point where they ask him permission to leave and in exchange they won't seek revenge on him. He agrees. The damage and shame the two sects suffered forces them to leave the area, and it becomes the protagonist's base of operations.

https://www.mangaupdates.com/series/mw8jsvc/the-reaper-inkyo

Could it be this one?

It is. Thank you.